The invention relates to a power steering apparatus which may be utilized to reduce the magnitude of a force which must be applied to steering means such as a steering wheel in order to steer a vehicle, and in particular, to an electrically driven power steering apparatus including an electric drive mechanism having an electric motor and which detects the magnitude of a torque that is applied from steering means to a direction controlling mechanism, which establishes a particular direction in which the vehicle is to run, and which applies a driving force of a magnitude corresponding to the magnitude of the torque to the direction controlling mechanism from the electric drive mechanism.
When it is desired to redirect wheels, a force of an increased magnitude is required to turn a steering wheel when the vehicle is at rest or is running at a low speed. In particular, with FF cars which are increasing in number recently and which have their front wheels designed as driving wheels, a further increase in the steering force is required.
To accommodate for this, a power steering apparatus which assists a driver in a steering operation has been proposed. Such apparatus produces a drive force in accordance with a steering force of a driver, and transmits it to a direction controlling mechanism (hereafter referred to as a steering system) which establishes a particular direction in which the vehicle is to run. Almost all of power steering apparatus which is currently in practical use is of hydraulic type. Thus, such apparatus is provided with a control valve, hydraulic cylinder and the like, and operates to produce an assisting steering force through a movement of a pressure oil in accordance with the steering force. However, it will be noted that such control valve, hydraulic cylinder and the like are bulky in size, and must be connected together through pipings which can only be bent with radii of curvature greater than a given value in order to avoid pressure losses. A hydraulic power steering apparatus must be provided with a reliable oil seal against leakage, which requires a troublesome mounting operation. Thus, a mounting of the hydraulic power steering apparatus presents a problem in a vehicle such as FF car where available remaining space is small.
To overcome the described problem, there has been proposed an electrically driven power steering apparatus which utilizes an electric motor as a drive source and which detects the magnitude of a torque applied to a steering system from steering means so that the motor applies an assisting steering force to the steering system in a manner corresponding to the magnitude of torque detected. With this arrangement, the space utility is improved, and in addition, a varying magnitude of assisting steering force which cannot be obtained with a conventional hydraulic power steering apparatus, for example, an assisting steering force which is dependent on a vehicle speed, may be developed when used in combination with an electronic controller.
In an electrically driven power steering apparatus of the kind described, an electric motor is utilized as a drive source, and accordingly, the power dissipation of the motor presents a problem. It is one of the features of the electrically driven power steering apparatus that an assisting steering force is available without requiring the operation of an engine to maintain the power steering apparatus operative upon shortage of a fuel or during the time a vehicle is being tracted. However, under such condition, an onboard battery is not charged, and accordingly the use of power by the power steering apparatus induces a premature degradation or a reduced voltage output from the onboard battery. It will be understood that a vehicle is equipped with a variety of electrical components in addition to the steering apparatus, including headlights, taillights and a car audio unit. Obviously, the power dissipation by the motor of the electrically driven power steering apparatus represents a major factor among these components. Accordingly, when a plurality of components are being operated and in addition the steering apparatus is operated, the total power dissipation may exceed the charging capability of the vehicle, resulting in a failure to properly charge the onboard battery even though the engine is in operation.
It will be understood that a degradation in the onboard battery leads to the inability to operate a starter, the inability to operate various electrical components or a misfire. Hence, it is necessary that a driver be fully careful about the total amount of the power dissipation onboard the vehicle whenever a voltage output from the onboard battery is reduced or when the engine is turned off. However, a driver will then be unduly stressed to determine a reduced voltage output from the onboard battery or a reduction in the charging capability. Accordingly, there has been no remedy to protect an onboard battery.